Wiggo is TdF champ, it wouldn’t look very impressive if he rode the Vuelta and got tonked by Contador et al who weren’t at the TdF.
The main reason though for Wiggo and Cav is that they are the big stars in Team Sky. Sky is a British team and this is their home tour. It’s a case of wheeling them out for the fans and getting good publicity for Sky.
More than one race occurs at the same time. A squad will have about 25 riders, a Grand Tour requires 9 riders per team, smaller tours or one day races will require 6, 7, 8 or 9 riders usually. Some races will suit other riders better than others, riders also need some time off racing for training and a bit of rest.
A team will plan for the whole season and will have a pretty good idea of who is doing what and who is treating what race as a major target. It’ll change slightly throughout the season.